I can try but if you've never played VR you would always underestimate these.
Aiming. On flat screen your gun is always perfectly aligned with your line of sight and your mouse controls both at the same time. The gun is also always straight. So aiming is quicker and more effective. In VR it's less quick and less precise because the gun can be oriented by the position of the controller and you need to coordinate head and hand movement like in real life. You need to train a bit to be good and the anyway you always lose the first 1 or 2 shots to calibrate your hand position which is not alway exactly where you expect it to be because of tracking accuracy (we're talking about millimeteres that have a huge impact on your aiming accuracy).
In a FPS this is a huge disadvantage.
Turning to watch your back: in flat screen just scroll your mouse on the side of few centimeters and you look and aim behind you at the same time and almost instantly. In VR you have the neck attacked to your head so you need to turn head+shoulder+body to look behind you (my head can roughly turn 110°), then you still need to coordinate look+hands to aim to your target. Don't forget also that the headset got some weight and cables attached so you can't turn back 180° every few seconds.
If you play roomscale it's physically more intense. I'm 38 and even if I did some karate and thai boxe when I was in the twenties, I can tell you I can't crouch and get up quickly so often as you would expect me to do with a simple CTRL button.
Performance under motion sickness. Expect most players performance to degrade during long and intense actions.
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